Insulated metal substrates have been used in the fabrication of electronic circuits to yield circuit board structures with thermal resistances lower than circuits formed on conventional alumina substrates. Typical metal substrate materials include steel, clad metal and aluminum alloys. Electrical insulation of metal substrates has been achieved using inorganic insulator coatings formed from such materials as porcelain enamels and glass dielectrics, and organic insulator coatings formed from such materials as KAPTON.RTM., TEFLON.RTM., acrylics, epoxies and phenolic polymers filled with ceramics.
Porcelain insulator coatings are fired at temperatures sufficiently high (e.g., about 600.degree. C. to about 900.degree. C.) to allow printing and firing of inorganic thick-film resistive, dielectric and conductive inks to form thick-film passive circuit devices and conductors. While effective in electrically insulating thick-film circuit elements from metal substrates, porcelain enamel coatings have coefficients of thermal expansion (CTEs) higher than conventional inorganic thick-film materials, which if not compensated for promotes thermal fatigue. (As used herein, the term "conventional inorganic thick-film materials" are those resistive, dielectric and conductive materials designed for use on alumina (e.g., 96%) substrates and characterized by sufficient process insensitivity, consistency, predictability and power dissipation to be suitable for high-volume applications, such as automotive electronic applications. Conventional inorganic thick-film resistive materials are also understood by this definition to have sheet resistances of up to at least 100 kilo-ohms/square.) To avoid a CTE mismatch, inorganic thick-film circuit elements formed on porcelain insulator coatings have been specifically formulated to be compatible with the particular porcelain material used for the insulator coating. Materials for inorganic thick-film circuit elements formed on porcelain insulator coatings must also tolerate the diffusion of constituents from these coatings, which would otherwise cause undesirable changes in the electrical and physical properties of the circuit elements. In other words, "nonconventional" inorganic thick-film materials must be used with porcelain insulator coatings, often yielding circuit elements whose process insensitivity, consistency, predictability and power dissipation are not as good as conventional inorganic thick-film materials.
Thick-film materials for circuit elements formed on glass dielectric insulator coatings must also be specifically formulated to be compatible with the particular glass material in order to tolerate diffusion of constituents from the insulator coatings. While glass dielectric insulator coatings may contain many of the same constituents present in an inorganic thick-film circuit element, the relative proportions of a given constituent may be such that interdiffusion during firing of the circuit element creates intermediate phases whose impact on the circuit element depends on the degree of diffusion and the proportion that the newly created phases assume relative to the total composition of the circuit element. It is not unusual for these new phases to alter the softening point, melting point, wetting characteristics and CTE of the circuit element, which are primary determinants of the electrical properties of thick-film passive circuit elements, including such properties as the sheet resistivity, temperature coefficient of resistance (TCR) and stability of a thick-film resistor.
In view of the above complications, it can be appreciated that circuit elements formed of conventional inorganic thick-film materials (as defined herein) have not been used on metal substrates. Instead, inorganic thick-film circuit elements on metal substrates have generally been limited to the use of specially formulated inorganic thick-film materials, and for such niche applications as camera flash bars and teapot heaters.
A significant limitation for the use of organic insulator coatings on metal substrates is their limited temperature capability, typically about 400.degree. C. Consequently, circuit elements formed of conventional inorganic thick-film materials that require firing at 600.degree. C. or higher are incompatible with metal substrates having an organic insulator coating. Instead, polymer thick-film (PTF) materials that cure at temperatures of up to about 300.degree. C. have been used to form the thick-film resistive masses and capacitive dielectrics of passive thick-film circuit elements. However, a significant drawback to this approach is the lower stability of PTF resistors as compared to inorganic thick-film resistors. For example, whereas inorganic thick-film resistors may exhibit a permanent resistance change of less than 1% when subjected to a harsh environment (e.g., 1000 hours at 150.degree. C.), a PTF resistor may exhibit a change of about 2% when subjected to the same environment. NiP is a "nonconventional" inorganic resistive material that is compatible with organic insulator coatings, but has a sheet resistance capability of only up to about 250 ohms/square, necessitating complex and/or large resistor designs to achieve suitable resistances for many applications. Finally, ruthenium-based thick-film materials taught by U.S. patent application Ser. No. 09/105,611 to Bowles et al. and U.S. patent application Ser. No. 09/178,758 to Ellis et al. are other examples of "nonconventional" inorganic thick-film materials that are compatible with organic insulator coatings for metal substrates.
In view of the above, it can be appreciated that the prior art lacks the ability to form thick-film resistors, capacitors and conductors on metal substrates from conventional inorganic thick-film materials. However, such a capability would be very desirable from the standpoint of processing, performance and reliability of a thick-film hybrid circuit.